Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is thereby imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reducing objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied on the wafer.
The performance of the projection exposure apparatus being used, however, is generally determined not only by the imaging properties of the projection objective but also by an illumination system which illuminates the mask. To this end, the illumination system contains a light source, for example a laser operated in pulsed mode, and a plurality of optical elements which generate light bundles, converging on the mask at field points, from the light generated by the light source. The individual light bundles desirably have particular properties, which in general are adapted to the projection objective.
These properties include among other things the illumination angle distribution of the light bundles which respectively converge on a point in the mask plane. The term illumination angle distribution describes the way in which the overall intensity of a light bundle is distributed between the different directions in which the individual rays of the light bundle strike the relevant point in the mask plane. If the illumination angle distribution is specially adapted to the pattern contained in the mask, then the latter can be imaged with high imaging quality onto the wafer covered with photoresist.
The illumination angle distribution is often not described directly in the mask plane, in which the mask to be projected is placed, but instead as an intensity distribution in a pupil plane which has a Fourier relation with the mask plane. This utilises the fact that each angle with respect to the optical axis, at which a light ray passes through a field plane, can be assigned a radial distance measured from the optical axis in a Fourier-transformed pupil plane. In the case of a so-called conventional illumination setting, for example, the region illuminated in such a pupil plane is a circular disc concentric with the optical axis. Each point in the mask plane is therefore struck by light rays at angles of incidence of between 0° and a maximum angle dictated by the radius of the circular disc. In the case of so-called unconventional illumination settings, for example ring-field (or annular), dipole or quadrupole illumination, the region illuminated in the pupil plane has the shape of a ring concentric with the optical axis, or a plurality of individual regions (poles) which are arranged at a distance from the optical axis. With these unconventional illumination settings, therefore, the mask to be projected can be illuminated exclusively obliquely.
With conventional illumination settings and ring-field illumination, the illumination angle distribution is typically rotationally symmetric in the ideal case. With quadrupole illumination, although the illumination angle distribution is ideally not rotationally symmetric, in the ideal case the poles in the pupil plane are typically illuminated so that the illumination angle distribution has a fourfold symmetry. Expressed more simply, an equal amount of light from all four directions therefore strikes a field point in the mask plane.
The symmetry properties of the respective illumination angle distribution can contribute to achieving dimensionally accurate imaging of the structures contained on the masks. In the event of deviations from these symmetry properties, for example, structures which are equally wide but oriented differently on the mask (for example vertically or horizontally) may be imaged with a different width on the photoresist. This can compromise unimpaired function of the microlithographically produced components.
To better quantitatively describe deviations from the aforementioned ideal symmetry properties of the illumination angle distributions, the term pupil ellipticity is often used. Expressed simply, the pupil ellipticity corresponds to the ratio of the amounts of light which strike a field point on the mask from orthogonal directions during an exposure. The more the pupil ellipticity deviates from 1, the more asymmetric is the illumination angle distribution.
Another property of the light bundles striking the mask plane is the telecentricity. The term telecentric illumination is used when the energetically central rays of the light bundle, which are generally referred to as principal or centroid rays, pass perpendicularly through the mask plane. With non-telecentric illumination, the entire light bundle strikes the mask to some extent obliquely. For the illumination angle distribution, this means that the different amounts of light come from opposite directions. In general, telecentric illumination is desired since the projection objectives are usually also telecentric on the object side. When correcting the pupil ellipticity, therefore, the telecentricity is desirably generally preserved.